The atomic mass of an atom (i.e., a specific isotope of an element) is measured in comparison with the mass of one atom of carbon-12 (¹²C) that is assigned a mass of 12 atomic mass units (amu). Atomic mass is sometimes erroneously confused with atomic weight—the obsolete term for relative atomic mass. Atomic weights, however, are still listed on many Periodic tables.

A mole of any element or compound (i.e., 6.022×10²³—Avogadro's number—atoms or molecules) weighs its total unit atomic mass (formerly termed atomic weight) in grams. For example, water (H₂O) has a molar mass (the mass of 6.022×10²³ water molecules) of approximately 18 grams (the sum of 2 hydrogen atoms, each with an atomic mass of 1.0079 amu, bonded with one oxygen atom with an atomic mass of 15.9994 amu).

In general usage if a specific isotope or isotope distribution is specified when...